In cooling towers, the water of a secondary condensation circuit is cooled, for example, mainly by evaporating a fraction of the water to be cooled, which water trickles over surfaces that are provided for this purpose and that are swept by a flow of air flowing in the opposite direction to the trickling, and it is cooled to a lesser extent by the convection that occurs on the heat exchange surface within the water itself.
These surfaces are carried by a heat exchange body, commonly referred to as “packing” and constituted by sheets made of PVC in particular, which sheets are assembled to one another so as to form a cellular structure. Each cell of the structure is in the form of a tube that is about 1.5 meters (m) long, with the mean dimension of its section being of the order of a few centimeters. The walls of the cells are fine (a few tenths of a millimeter) and they are pierced by numerous orifices.
The heat exchange bodies are suspended in the cooling tower between an installation for sprinkling the water that is to be cooled and a bottom basin for recovering the cooled water, droplet separator or capture means also being located higher up in the tower in order to retain as much as possible the water droplets that are entrained by the cooling air. It is important not to exhaust this water into the atmosphere since it can carry germs that proliferate readily in the tower since it operates at a temperature that encourages such proliferation.
In use, the heat exchange body and the droplet separators become covered in a deposit of mineral salts because of the evaporation that occurs on the trickling surfaces. This deposit grows over time and, in the packing, it can reach a weight that is as much as ten times the weight of the body itself in certain installations. This deposit presents numerous drawbacks: it constitutes an obstacle to trickling and thus to the effectiveness of heat exchange, it constitutes a nest for retaining various germs that are present in the water under conditions that encourage their proliferation, it constitutes a very significant extra load on the structure supporting the heat exchange body since it is generally suspended inside the tower, . . . .
At least in theory, there are several ways of remedying that drawback. One consists in chemically treating the water for cooling so as to remove salts therefrom, thereby avoiding scaling of the heat exchange surfaces. That cannot be envisaged in the cooling towers of power stations, whether nuclear or fossil fuel.
It is also possible to dissolve the scale chemically using appropriate solutions. That technique raises difficult problems of effluent treatment and therefore has an economic impact on operating costs.
Finally, it is possible to envisage shaking the heat exchange body with any appropriate mechanical means; on being tested, that method has led to so much deterioration of the heat exchange body as to render it practically unsuitable for subsequent use.
Finally, it is possible to clean the heat exchange body mechanically after it has been disassembled, but that constitutes an operation that is extremely expensive given the large volume of the body (10 cubic meters (m3) to 12,000 m3 in units of about 2 m3.